American researchers say they have found new genetic evidence about the family tree of mammals.

They say it proves that marsupials - the pouched mammals found mainly in Australia - are more closely related to most other mammals, including humans, than had been thought.

Australia's famous marsupial: The koala

They also say previous genetic research about mammalian evolution may be inaccurate, which could shed new light on our own descent from ape-like ancestors.

The work questions the widely held view that modern humans evolved in Africa, then conquered the rest of the world.

There are three groups of mammals. They comprise:

Marsupials, which raise their young in pouches, such as the kangaroo and the koala

Monotremes, which lay eggs and are represented now only by the duck-billed platypus and the spiny anteater

Placental mammals - all the rest, including dogs, horses, and ourselves.

Four years ago Chinese and Swedish researchers said they had found that marsupials and monotremes were close relatives.

The new evidence came from what is called mitochondrial DNA - genetic material from outside the nucleus of the cell.

Fossil evidence

Earlier this year, researchers who studied fossilised mammal teeth suggested the opposite was the case - that, many millions of years ago, monotremes evolved in the Southern Hemisphere, while marsupials and placentals shared a more recent ancestor in the Northern Hemisphere.

Professor Randy Jirtle and his team at Duke University in North Carolina, US, now say they have analysed evidence from cell-nucleus genes that shows the teeth were telling the truth. And he believes analysis of mitochondrial DNA may not be so reliable in explaining mammalian evolution.

That may have implications for another big evolutionary debate: how humans came to be. Mitochondrial DNA has provided support for the view that modern humans evolved in Africa and then spread out through the rest of the world.

If that support now crumbles, advocates of the other major theory of how we came about - that we evolved gradually in many regions at the same time - will consider their case to be stronger.